This invention relates to systems and methods that destroy active species of pre-cursor gases used in atomic layer deposition (ALD). More particularly, this invention relates to systems and methods that destroy active species of pre-cursor gases inside the reactor in which the ALD occurs.
ALD is a process in which atoms are deposited on a substrate, monolayer at a time, to grow a multi-layered thin film. In typical ALD processes, a first pre-cursor gas enters a reactor in which a substrate is placed. The first pre-cursor gas saturates the surface of the substrate forming a first monolayer. The reactor is then typically purged with an inert gas. Purging is required to prevent parasitic chemical vapor deposition (CVD) reactions from occurring between the first pre-cursor gas and a second pre-cursor gas that enters the chamber after purging. If most of the pre-cursor gas is not purged from the chamber, these reactions can form particles that will either ruin the thin film being grown, result in an undesired CVD-type deposition, or both. Problems associated with CVD-type deposition include poor step coverage and poor uniformity.
After the first pre-cursor gas is purged, a second pre-cursor gas enters the reactor and reacts with the adsorbed monolayer or partial monolayer (i.e., the first monolayer) to form a monolayer of the desired film. The reactor is again purged with an inert gas. This process is repeated until the thin film is grown to a desired thickness.
In some ALD reactions, one of the pre-cursors may include an active species. An active species is a molecule that is not in its most stable state. Active species will readily react to either accept or donate electrons in a reaction (e.g., O3, O* or N* (where * denotes an excited state with excess energy)). Because most active species readily decay or recombine into a more stable state as temperature increases, they have a relatively short half-life in an ALD chamber operating at temperatures in excess of 400° C. However, in some cases, it may be advantageous to grow a film via ALD at lower temperatures such as 200° C. In this situation, the half-life of the active species is increased, and for a given purge time, there is a higher initial concentration of active species present to be purged from the reactor. This directly reduces the throughput of the reactor.
Variables that can affect the half-life of active species of gases include temperature and pressure. As temperature and pressure increase, the half-life of the active species decreases because of simple kinetics. Destruction of the active species is necessary because of their high reactivity. For example, an active species such as ozone can irritate the eyes and lungs, and so should be completely converted into O2 before being discharged to the atmosphere. ALD processes that require the use of an active species typically run at temperatures high enough to destroy (e.g., consume or decompose) most of the active species before they leave the chamber. This reduces the concentration level of the active species significantly.
However, current experience with ALD processes indicates that it may be advantageous to operate at temperatures lower than the high temperatures of many known ALD processes. For example, many of the metal-organic pre-cursors that may be used in the ALD of hafnium (Hf) oxide decompose at temperatures higher than 200° C. Therefore, in order to obtain a film with acceptable properties (e.g., step coverage, low carbon content) the reactor must be operated at a temperature close to or lower than 200° C. When ozone is used as an oxidizer in this process, these lower temperatures are not sufficient to decompose all of the ozone before it leaves the reactor.
For example, in the ALD of hafnium oxide, if ozone is supplied to a reactor and the chamber is running at 400° C., a certain percentage of the ozone, X, will be consumed by the reaction with the Hf pre-cursor and another percentage, Y, will be consumed by kinetic recombination of the O3 and O* to form O2. This kinetic recombination reaction is driven by temperature. The overall percentage of ozone remaining in the chamber will be Z. However, if the chamber is running at 200° C., the percentage of ozone that recombines will be far less than Y. Thus, the amount remaining to be purged will be greater than Z. This requires a longer purge time.
In view of the foregoing, it would be desirable to decrease the purge time of active species of pre-cursor gases from reactors used in ALD to improve throughput.